An easy way to get this one is right after you obtain the Fire Ammo Blueprint. Use the crafting station nearby to make as much Fire Ammo for the Assault Rifle as possible. Equip your Assault Rifle and also equip Fire Ammo for it in the inventory. Save your game just in case you don't get the achievement on your first try. Now exit the room and you should encounter a Seeker. Shoot him with your Assault Rifle and it will die quickly, Next turn on your flashlight and shine it on the pods that are in the middle of the room where the Seeker spawned. Two scavenger's will pop out. After you kill them, run and up ahead the floor will have collapsed and 3 more scavengers will have spawned down below. Shoot two of them and the achievement will pop.

If this doesn't work for you, you can also find a Hive Opening and shine the flashlight into it and you can get it this way aswell. If you go back to the tunnels section of the ship the Hive Openings there don't spawn Heavies.